The present invention generally relates to an electrical resistor and more particularly, to a variable resistor having a resistant base plate or substrate formed by molding of resin material and terminal members integrally molded in said substrate.
Variable resistors, especially those of small sizes are widely employed in various electrical and electronic equipment and devices, for example, compact radio and TV sets, VTR, audio set, etc., and there has been an increasing demand for such variable resistors which are highly reliable in performance and yet, can be readily manufactured on a large scale through simple processings.
Generally, the variable resistors of the above described type includes a base plate or substrate integrally molded with terminal members which are arranged to be exposed, in the vicinity of forward ends thereof, to the surface of the substrate, a resistant film or resistant layer formed on the surface of said substrate so as to cover the exposed portions of said terminal members, and a contactor or slider movably mounted on said substrate for sliding movement over the resistant layer to adjust the resistance value. The terminal members for the variable resistor of the above described type are required to have a sufficient solderability for mounting the variable resistor onto a printed circuit board, etc. as in other types of variable resistors, and are generally constituted by a metallic material, for example, brass and the like, with a precious metal such as silver, etc. being plated thereon. Although the terminal members plated by the precious metal as described above are superior in characteristics, not being readily subjected to oxidation, with a high electrical conductivity, there has been such a disadvantage that the high cost of the plating material employed runs counter to the recent trend to save precious metal materials. Accordingly, in the similar method as in the other types of variable resistors, it may be conceived to replace the terminal members plated with the precious metal, by those plated, for example, with solder of Sn-Pb alloy. In connection with the above, as a result of various performance tests made by the present inventor on sample variable resistors which are actually prepared with the employment of resistant substrates of resin material having solder-plated terminal members integrally molded therein, it was noticed in a certain special test that the resistant layer was slightly raised or floated at its portion covering the exposed portions of the terminal members so as to be readily separated or peeled off thereat. Based on the results of experiments conducted from various angles, it has been concluded that the phenomenon as described above is attributable to the fact that, since the temperature for baking the resistant layer to the substrate is arranged to be higher than a melting point of plated solder layer, the solder is melted during the baking of the resistant layer, and thus, the resistant layer is undesirably raised through the molten solder. Therefore, the rising or floating of the resistant layer may be prevented by lowering the baking temperature through proper selection of material for the resistant layer, but even in the above case, similar disadvantage was encountered during soldering for attaching the variable resistor to a printed circuit board or the like. The inconvenience as described above may be ascribed to the phenomenon that, due to the conduction of heat during soldering to the exposed portions of the terminal members through the metallic material of said terminal members, the solder for plating in such portions is undesirably melted, but such drawbacks as described above can not be solved by the selection of metallic materials for the terminal members, etc.